Cu²⁺ catalyzed benzoxazole formation from Schiff base: Reactive organic chemosensors and effect of hydroxyl position

Triphenylamine (TPA)‐based Schiff base chemosensors (( E )‐2‐((4‐(diphenylamino)benzylidene)amino)phenol [ TPA‐AP ], ( E )‐5‐(diphenylamino)‐2‐(((2‐hydroxyphenyl)imino)methyl)phenol [ HTPA‐AP ], and ( E )‐5‐(diphenylamino)‐2‐((phenylimino)methyl)phenol [ HTPA‐A ]) were synthesized and explored the role of metal chelating hydroxyl functionality position for heavy metal ion sensing. TPA‐AP and HTPA‐AP showed Cu 2+ ion‐induced chemical transformation and produced oxazole derivatives with turn‐on fluorescence. In contrast, HTPA‐A exhibited metal chelation‐induced turn‐on fluorescence for Zn 2+ ion. The reactive chemical sensing of TPA‐AP and HTPA‐AP led to high selectivity of Cu 2+ , and other ions did not show any interference. The concentration‐dependent studies of Cu 2+ and Zn 2+ indicated the limit of detection (LOD) up to 6.07 × 10 −7 and 4.6 × 10 −7 M, respectively. Further, Cr 3+ , Fe 3+ , and Fe 2+ coordination with TPA‐AP and HTPA‐AP exhibited visible color change from colorless to yellow. However, the yellow color was disappeared and becomes colorless due to imine hydrolysis. Interestingly, all three metal ions showed different rate of hydrolysis and color disappearance. TPA‐AP with Fe 3+ exhibited fast hydrolysis and immediate color change whereas Fe 2+ and Cr 3+ required longer time. But HTPA‐AP and HTPA‐A produced stable complexes with Cr 3+ , Fe 2+ , and Sn 2+ and Fe 3+ alone showed hydrolysis. Thus, the present studies provide structural insight for designing reactive fluorescence and colorimetric Schiff base chemosensor for heavy metal ions.